The invention claimed in this application is an improvement over previous processes for the C-6 epimerization of penicillin sulfoxides. Specifically, the process of this invention is a highly efficient process (i.e., 80% yields or greater) employing inexpensive, readily available reagents, (i.e., trimethylsilyl chloride and triethylamine). Although H. Vanderhaeghe et al., Journal of the Chemical Society, Perkin Transactions I, 932, 1973, describes the epimerization of 6.beta.-phenylacetamidopenicillin-1.beta.-sulfoxide to the corresponding 6.alpha. isomer in 87% yield, the process is carried out with N,O-bis(trimethylsilyl)-acetamide (BSA) and 1,5-diazabicyclo [4.3.0]-non-5-ene (DBN) as the reagents, and these reagents are more expensive than the triethylamine and chlorotrimethylsilane used in the present process. Alternatively, Vanderhaeghe et al., Journal of the Chemical Society, Perkin Transactions I, 937, 1973 describes the epimerization of benzyl 6.beta.-phenoxyacetamidopenicillanate to the corresponding 6.alpha. isomer using trimethylamine and BSA, but only a 38% yield was obtained. The present process overcomes the dual problems of expensive reagents and/or low yields as exemplified by the above two references.